1. Field of the Invention
The present invention is directed to rotary transformers, and more particularly to a rotary transformer with a synchronized mode of operation that transfers both power and data between two structures.
2. Description of the Related Art
Rotary transformers are often used for transmitting both data and power between two structures that rotate relative to one another, such as between a vehicle tire and its corresponding wheel axle in a tire pressure sensor system. In another example, a rotary transformer can be used to couple data and power from a steering column to a steering wheel, as disclosed in co-assigned U.S. Pat. No. 6,121,692, the entire contents of which are herein incorporated by reference.
As is known in the art, loosely coupled power transformers do not conduct power efficiently between the primary and secondary of the transformer. Instead, a part of the input current into the primary coil stores energy in the leakage. inductance of the coil. Prior art structures often include a Zener diode across the primary to absorb the energy of the voltage spike that occurs in the transformer when the current to the primary coil is turned off. More particularly, the Zener diode will conduct current before the drive transistor in the primary side breaks down. However, under this approach, the stored energy is dissipated as heat, thereby wasting the energy built up in the primary coil""s leakage inductance and lowering the power coupling efficiency of the transformer.
To overcome this problem, conventional rotary transformer designs tend to focus on methods of increasing the coupling efficiency by constructing a magnetically efficient structure for power transmission, such as by using more expensive, high-efficiency core materials, and then adding a complex load impedance mechanism for providing limited two-way communication through the transformer. This results in an overly complicated structure requiring close mechanical tolerances, which increases the manufacturing cost of the system. Further, the bandwidth for these structures tends to be relatively narrow, which limits the amount of data or the speed at which data can be transmitted between the primary and secondary sides of the transformer.
To increase the bandwidth in the rotary transformer, a loosely coupled rotary transformer that includes a resonant circuit, such as a resonating capacitor connected to a power MOS transistor, may be coupled across the primary coil of the transformer, as described in co-pending, co-assigned U.S. patent application Ser. No. 09/395,817 filed on Sep. 14, 1999, the entire contents of which are herein incorporated by reference. In the loosely coupled rotary transformer, the resonant circuit is connected and disconnected from the transformer during a power transfer mode and a data transfer mode, respectively. During the power transfer mode, stored energy in the leakage inductance of the primary coil is used for the power coupling, via the resonant circuit, instead of being dissipated as heat. The resonant circuit is disconnected from the rotary transformer during the data transfer mode to maximize bandwidth for two-way data transfer between the primary and secondary sides of the transformer. Including the resonant circuit in the loosely coupled transformer optimizes data and power transfer without requiring the use of high-cost, high-efficiency magnetic structures in the core of the transformer.
The loosely coupled rotary transformer utilizes a fixed frequency drive circuit, and a resonant drive mode that is very power efficient compared to other known rotary transformer drive methods. The transformer resonant frequency and drive frequency is matched for the nominal supply voltage and secondary load. However, a problem may arise when the supply voltage is not properly regulated, or the secondary load is subject to large changes from a nominal level. In both cases, the power coupling efficiency may decrease from a nominal level.
The inventors of the present invention have recognized this problem and have modified the operation of the rotary transformer drive circuit to maintain high power efficiency for changes in either supply voltage or secondary load. This is especially important for vehicle operation where the supply voltage for proper operation may vary between a voltage of approximately 9.0 and approximately 16.0 volts, which is almost a 2:1 ratio.
The invention comprises a rotary transformer with a synchronous mode of operation to facilitate the transfer of power and two-way communications between two structures, such as a column and steering wheel of a vehicle. During normal operation, the rotary transformer repetitively alternates between a power transfer mode and a data transfer mode by multiplexing time across the rotary transformer. A microprocessor supplies a pulse train that periodically applies fill power from a power supply, such as a vehicle battery to the transformer""s primary coil, or xe2x80x9ccolumn coil.xe2x80x9d In the referenced prior patent, during the power mode when the pulses supplied to the primary coil are xe2x80x9conxe2x80x9d, the microprocessor disconnects a resonating or tuning capacitor C1 from the primary coil. When the pulses are xe2x80x9coffxe2x80x9d, the resonating capacitor is reconnected, at which time energy stored in the resonating capacitor is supplied across the rotary transformer. By connecting the resonating capacitor C1 to the primary coil only when the pulses are turned xe2x80x9coffxe2x80x9d, the power required to drive the rotary transformer is minimized and the energy recovered from the primary coil is maximized. For synchronized operation, the subject of this application, the tuning capacitor is connected during the entire power mode. It is disconnected only during the data transfer mode. The microprocessor can also adjust the width of the pulses supplied to the primary coil to maintain a constant power level at the wheel circuit using means well known in the art, such as a voltage regulator.
After a preset length of time allotted for the power transfer mode, the microprocessor causes the primary circuit to change to the data transfer mode. During this mode, the primary circuit transmits a preset number of data bits to the secondary side across the rotary transformer, and then the secondary side transmits a preset number of bits to the primary circuit across the rotary transformer. Then, the circuit returns to the power transfer mode and repeats the sequence.
One aspect of the invention is that during the power transfer mode, the drive transistor of the coil drive circuit is switched xe2x80x9conxe2x80x9d when the voltage across the primary coil changes from positive to negative at approximately one half of a cycle to provide a synchronous mode of operation. This synchronized mode of operation virtually eliminates a current spike through the diode of the coil drive circuit that exists using conventional modes of operation, which turn the drive transistor xe2x80x9conxe2x80x9d at the end of the power transfer mode, such as in fixed frequency and variable frequency modes of operation. By preventing the current spike through the diode of the coil drive circuit, the stress to the driving transistor and electromagnetic interference to the operating environment of the rotary transformer are minimized. In addition, the synchronous mode operation of the invention provides the microprocessor a sufficient amount of time to recognize the change of the resonant waveform of the primary circuit during the power transfer mode so that it can change the output ports, unlike conventional modes of operation.